WASHINGTON -- Global warming tussles erupted Thursday on Capitol Hill and at the White House, where the top spokesman was grilled about President Bush's early stance on the causes of climate change.

Spokesman Tony Snow fielded questions about an "open letter" released Wednesday by two White House officials complaining that some media stories inaccurately described Bush as coming late to the idea that human activities spur global warming.

The letter included a quote from a Bush speech in 2001, in which he cited a National Academy of Sciences report that said climate change was "due in large part to human activity."

The U.S. government has refused to sign the Kyoto Protocol, which sets ambitious targets to reduce greenhouse gases that are believed to contribute to rising global temperatures and changing weather patterns.

The United States is responsible for one-quarter of the world's emissions of carbon dioxide, a major greenhouse gas.

Snow gave a fuller excerpt from Bush's 2001 speech, which dwelled on the uncertainties of the science on climate change, natural fluctuations in climate and the open question on the possible impact that various human actions have on it

A report on global warming released Feb. 2 by the Intergovernmental Panel on Climate Change stated with 90 percent certainty that humans are the main cause.

House Speaker Nancy Pelosi, a California Democrat and environmentalist, cited the report in testimony before the House Science and Technology Committee, taking aim at Republicans in Congress who were in the majority until November.

"For 12 years, the leadership in the House of Representatives stifled all discussion and debate of global warming," she said. "That long rejection of reality is over, to the relief of members, I believe, on both sides of the aisle."

NO 'RUNNING ROUGHSHOD'

Pelosi called for mandatory caps on greenhouse gas pollution, which the White House and many Republicans reject.

She said she hoped to have legislation on global warming and energy independence ready by July 4, which she called Energy Independence Day.

Pelosi preceded four scientists who worked on the report, and like them was compelled to answer the committee's questions -- a departure from a tradition in which fellow members of Congress are excused from taking questions after testimony.

The issue was forced by Rep. James Sensenbrenner, a Wisconsin Republican, who asked, "What are you planning to do, Madame Speaker, to make sure that we don't legislate on this area in a way that wrecks our economy and costs our workers jobs?"

Pelosi said technological innovations and bipartisan cooperation would be needed to confront the problem.

"This isn't about running roughshod," she said. "This is about working together."

"I think the world's a bit perplexed at our very slow response to this threat," Boxer said at a hearing of the Senate Foreign Relations Committee.

Rice replied that the Bush administration had partially funded the intergovernmental report and spent $5.8 billion a year on the issue.

"Yes, I believe that global warming is a problem," Rice said. "We ought to be very active and we are."

U.S. Energy Secretary Sam Bodman told the House Energy and Commerce Committee the Bush administration would continue to oppose a mandatory limit on U.S. greenhouse gas emissions.

But Bodman said he would work with Congress to craft a bill to fight global warming. (Additional reporting by Tom Doggett) (Editing by John O'Callaghan; Reuters Messaging: deborah.zabarenko.reuters.com@reuters.net; 202 898 8388))